The Arctic Summer Cloud Ocean Study, ASCOS, was a six-week expedition in August-September 2008 north of the 87th latitude onboard the Swedish icebreaker Oden. The expedition was a contribution to the International Polar Year 2007/2008. ASCOS studies some of the controlling factors of the low-level cloud system, especially the formation of cloud condensation and ice nuclei, over the Arctic pack ice. This will improve the accuracy of the climate models in the Arctic area where the climate is expected to change faster than in any other part of the world. A distinct feature of ASCOS is its necessarily interdisciplinary nature, which includes physical oceanography, marine and sea ice biogeochemistry, surface microlayer chemistry, aerosol, gas-phase and cloud chemistry and physics, and meteorology. The expedition sailed from Longyearbyen, Svalbard, in the beginning of August. After a week's cruise the icebreaker was moored to a several square kilometres wide ice floe north of the 87th latitude. The ship was drifting with the ice floe for a month before the return voyage to Svalbard in the early September. The Finnish contribution to the experimental part of ASCOS consisted of researchers and instruments from the Finnish Meteorological Institute (FMI) and the University of Kuopio (UKu). The FMI's measurement programme consisted of meteorology, ozone and radioactivity soundings, a UV radiation sensor, a visibility meter, a ceilometer, filter sampling for radionuclide, heavy metal and PAH analyses, canister sampling for VOC analyses, fog droplet and aerosol particle size distribution measurements, an ozone monitor, and a sea water bubble camera. The UKu measurement programme consisted of ultrafine particles organic fraction measurements.